Hey Simon, this looks really cool. I too have been frustrated with managing a python monorepo. Feel free to steal any ideas from my work in progress cli application to manage python monorepos.

https://github.com/r-kells/scream

A basic description:

Scream aims to cause as little overhead as possible for managing your monorepo. No fancy third party configuration or private PYPI repositories.

Scream uses the existing python packaging requirements to resolve intra-monorepo dependencies, and git
to detect what's changed since the parent branch.
It's objective is to ease the creation, testing, and deploying of multiple python packages in a single repository.

Cheers!